examples/l2fwd-cat: fix build

<ctype.h> and <errno.h> need to be included for the build
since they were removed from <rte_common.h>.

examples/l2fwd-cat/cat.c: In function ‘parse_set’:
examples/l2fwd-cat/cat.c:66:16:
   warning: implicit declaration of function ‘isblank’
   66 |         while (isblank(*str))
      |                ^~~~~~~
examples/l2fwd-cat/cat.c:18:1:
   note: include ‘<ctype.h>’ or provide a declaration of ‘isblank’
   17 | #include "cat.h"
  +++ |+#include <ctype.h>
   18 |
examples/l2fwd-cat/cat.c:70:15:
   warning: implicit declaration of function ‘isdigit’
   70 |         if ((!isdigit(*str) && *str != '(') || *str == '\0')
      |               ^~~~~~~
examples/l2fwd-cat/cat.c:70:15:
   note: include ‘<ctype.h>’ or provide a declaration of ‘isdigit’
examples/l2fwd-cat/cat.c:75:17:
   error: ‘errno’ undeclared (first use in this function)
   75 |                 errno = 0;
      |                 ^~~~~
examples/l2fwd-cat/cat.c:18:1:
   note: ‘errno’ is defined in header ‘<errno.h>’;
   did you forget to ‘#include <errno.h>’?
   17 | #include "cat.h"
  +++ |+#include <errno.h>

Fixes: 72b452c5f2 ("eal: remove unneeded includes from a public header")

Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>

examples/l2fwd-cat/cat.c

@@ -2,6 +2,8 @@
  * Copyright(c) 2016 Intel Corporation
  */
 
+#include <ctype.h>
+#include <errno.h>
 #include <getopt.h>
 #include <inttypes.h>
 #include <limits.h>